A modulated optical data signal in a fiber optic communication network is subject to degradation as the signal passes through an optical fiber from an optical transponder at a data source to another optical transponder at a data destination. Attenuation and dispersion are two forms of degradation known to affect optical data signals. Attenuation refers to a form of signal degradation in which some of the light injected into an optical fiber by a source transponder does not arrive at a destination transponder. A measurement of attenuation from the optical fiber may be made by, for example, making a measurement of the optical power of an optical data signal at an input to the optical fiber, making another measurement of optical power at an output from the optical fiber, and comparing the two optical power measurements. Dispersion refers to a form of signal degradation in which an optical pulse detected by a receiver has been distorted by its passage through the fiber optic communication network into a broader, more rounded shape compared to the shape the pulse had when it was output from a transmitter. Undesirable effects of dispersion include uncertainty in measurements related to time, for example, uncertainty in synchronizing an optical pulse representing digital data with a clock signal in a receiver circuit, and uncertainty in determining the time when a signal changes from one logical state (e.g., “true” or “1”) to the opposite state (e.g., “false” or “0”).
Equipment in the signal path between the source transponder and the destination transponder further degrades the optical data signal by adding noise to the signal. There are many potential noise sources, for example noise from adjacent channels in Wavelength Division Multiplexing (WDM) systems, noise from attenuators used to equalize power among adjacent channels in Dense Wavelength Division Multiplexing (DWDM) systems, and noise from amplifiers. An amplifier not only introduces its own noise into the optical data signal, for example amplifier spontaneous emission noise, it also amplifies any noise already present in the optical data signal. Each additional stage of amplification between the source transponder and destination transponder may therefore increase the difficulty of recovering data from the optical data signal.
Noise accumulates in the optical data signal as the signal travels from a source transponder to a destination transponder. A measurement of noise referred to as an optical signal to noise ratio (OSNR) represents a ratio between the amplitude of a desired portion of an optical data signal, the desired portion corresponding to data to be transmitted over a fiber optic network, and the amplitude of noise in the optical data signal. In general, data may be recovered with fewer communication errors, that is, fewer differences between transmitted data and received data, from an optical data signal having a high OSNR value than from one with a low OSNR value. At a sufficiently low value of OSNR, the communication system may be unable to recover transmitted data from an optical data signal.
A measurement of communications system reliability referred to as the bit error rate (BER) expresses a ratio between the number of differences between transmitted bits and received bits and the total number of transmitted bits. A low BER is more desirable than a high BER. A mathematical relationship is known between BER and OSNR in which a low BER value corresponds to a high OSNR value and a high BER value, that is, a relatively large number of differences between transmitted and received bits, corresponds to a low OSNR value. Communication systems increase system reliability by minimizing BER.
A communication system may attempt to minimize BER by adjusting a numerical value for a transponder parameter referred to as a decision threshold voltage (Vdtc). A transponder compares a reference voltage Vref, where Vref is related to Vdtc, to an amplitude of an incoming optical data signal to determine if the optical data signal at the time of comparison has an amplitude representative of a logical “1” or a logical “0”. For each selected value of Vdtc there is a corresponding reference voltage Vref and a corresponding measured value of BER. An optimal value of Vdtc, referred to as Vdtc_opt, corresponds to a minimum value of BER. Some communication systems known in the art use a Vdtc adaptive search algorithm to find a value of Vdtc corresponding to Vdtc_opt, further corresponding to a minimum value of BER.
A mathematical relationship between Vdtc and BER may be illustrated by plotting a curve comprising (Vdtc, BER) data pairs at a selected combination of values for input signal optical power RXP and OSNR. Furthermore, a family of related curves may be plotted to show BER as a function of Vdtc for more than one pair of values of RXP and OSNR. FIG. 11 shows an example of a family of four related curves for measurements made on a communication system known in the art. Each curve in FIG. 11 is a plot of measured BER values as a function of Vdtc for a pair of values of RXP and OSNR measured from an optical data signal.
In the prior art illustration of FIG. 11, curve A is related to an optical signal having low OSNR and high RXP, curve B is related to an optical signal having low OSNR and low RXP, curve C is related to an optical signal having high OSNR and low RXP, and curve D is related to an optical signal having high OSNR and high RXP. On each curve there is a value of Vdtc corresponding to a minimum value of BER, as indicated by dashed lines in FIG. 11. Vdtc corresponding to minimum BER for curve A is labeled Vdtc_optA, Vdtc corresponding to minimum BER for curve B is labeled Vdtc_optB, and so on. Note that in the example of data from a prior art communications system shown in FIG. 11, Vdtc_optB and Vdtc_optC have approximately the same value (about 0.42 on the Vdtc axis). Vdtc_optB and Vdtc_optC may have different values for data collected from another transponder or for data collected under another combination of OSNR and optical power.
A Vdtc search algorithm running in a communication system would be expected to find a value of Vdtc corresponding to Vdtc_optB for an optical signal having low RXP and low OSNR. The Vdtc search algorithm would similarly be expected to find the other labeled Vdtc_opt points under the conditions of RXP and OSNR applying to each of the remaining curves in FIG. 11. However, for communication systems known in the art, a Vdtc search algorithm may not converge to a minimum value of Vdtc if BER is greater than a BER threshold value during the search. For example, a Vdtc search algorithm operating in some communications systems known in the art will be unable to converge to a Vdtc result for a BER greater than a BER threshold value of 0.001. A BER threshold value of 10−3 (0.001) is labeled in FIG. 11.
After a Vdtc search algorithm converges on a numerical value of Vdtc corresponding to Vdtc_opt for a particular set of OSNR and RXP values, a reference voltage Vref having an amplitude related to the numerical value of Vdtc is determined. A reference voltage Vref related to a decision threshold voltage Vdtc corresponding to Vdtc_opt is referred to as Vref_opt. In communication systems known in the art, a determination of a value for Vref from a value of Vdtc, also referred to as mapping Vdtc to Vref, is made using a linear relationship between Vdtc and Vref. A linear mapping may be accomplished by relating Vref to Vdtc with the linear relationship expressed in equation (1):Vref=(c1×Vdtc)+c2   (1)where c1 and c2 are mathematical constants and c2 is added to the product of c1 and Vdtc to determine a value of Vref.
In the absence of signal degradation, Vdtc would be expected to have a value corresponding to halfway between an optical data signal amplitude representing a logical “1” and an optical data signal amplitude representing a logical “0”. However, a search algorithm for Vdtc may take into account a well known observation that noise generally affects the “on” state of an optical data signal more than the “off” state of the signal. The resulting value of Vdtc may therefore be less than halfway between the optical data signal amplitude representing a logical “1” and the optical data signal amplitude representing a logical “0”.
A Vdtc search algorithm in a communication system known in the art would be expected to converge to an optimum value of Vdtc for an optical data signal having stable OSNR and RXP as long as BER is less than the BER threshold value. A Vdtc search algorithm may fail to converge when BER is greater than the BER threshold value. When the measured values of RXP and OSNR of an optical data signal change, a related value of Vdtc_opt also changes. Communication systems known in the art may be unable to find Vdtc corresponding to Vdtc_opt for rapid or large changes in OSNR and RXP, at least in part due to their use of linear mapping as in equation (1). Linear mapping in communication systems known in the art is known to have at least two important shortcomings. For example, individual Vdtc_opt values corresponding to different combinations of RXP and OSNR measured from an optical data signal may be misaligned, that is, a Vdtc_opt value for one combination of OSNR and RXP may be substantially different from a Vdtc_opt value for a different combination of RXP and OSNR. A misalignment of Vdtc_opt points is illustrated in the prior art example of FIG. 11, wherein Vdtc_optA, Vdtc_optB, and Vdtc_optD have substantially different values along the Vdtc axis.
Another shortcoming of linear mapping is that BER is especially sensitive to Vdtc values computed for an optical data signal having low optical power. That is, at low optical power, a relatively small change in Vdtc can cause a relatively large change in BER. As shown in FIG. 11, a sensitivity of BER to changes in Vdtc is related to a span of each curve, where a span of a curve is defined as a difference in Vdtc values for two points on the curve at a selected value of BER. For example, in FIG. 11, Vdtc_optA has a much larger span than Vdtc_optB or Vdtc_optC at the same BER threshold value, and the span of Vdtc_optD is greater than the span of Vdtc_optA. In FIG. 11, BER has less sensitivity to a change in Vdtc on curves A and D, the curves related to an optical signal with high optical power, than for curves B and C, the curves related to an optical signal with low optical power. In general, a curve with a narrow span will have higher sensitivity of BER to Vdtc than a curve with a wider span.
A misalignment of Vdtc_opt values is undesirable in part because communication systems known in the art are limited by the use of linear mapping to adjust Vdtc along a continuous transitional path between a previous Vdtc_opt value and a new Vdtc_opt value resulting from a change in OSNR, a change in RXP, or changes in both OSNR and RXP. A misalignment between old and new Vdtc_opt values may cause a Vdtc search algorithm to converge slowly or to fail to converge. For example, a prior art communication system responding to a change in OSNR or optical power may seek Vdtc along a transitional path between misaligned old and new Vdtc_opt values on which BER increases or even exceeds the BER threshold value, in which case the Vdtc search algorithm fails to converge.
A transition between old and new Vdtc_opt values is more likely to cause a Vdtc search algorithm to fail when a change in the optical data signal is large, the change occurs over a relatively short time duration, or the old and new Vdtc_opt values are widely separated. In the context of determining a decision threshold voltage Vdtc and its related reference voltage Vref, a relatively short time duration refers to a time duration that is less than an amount of time needed for a Vdtc search algorithm to converge to a new value of Vdtc. One can appreciate that a Vdtc search algorithm will output non-optimal values of Vdtc, and the transponder will accordingly use a non-optimal value of Vref to recover data from an optical data signal, if the algorithm takes longer to converge than an amount of time in which OSNR or RXP change enough to make a previous value of Vdtc non-optimal. For example, a change in an optical data signal from a high value of RXP and a high value of OSNR to a low value of RXP and a low value of OSNR may cause a Vdtc search algorithm to fail before a communication system can settle into a new Vdtc corresponding to a new Vdtc_opt. In the example of measurements from a prior art communication system in FIG. 11, a change in an optical data signal from high OSNR and high RXP, for which a Vdtc search algorithm is expected to locate Vdtc equal to Vdtc_optD, to low OSNR and low RXP, will cause Vdtc to change to a value for which BER is greater than the BER threshold value before Vdtc reaches Vdtc_optB. When BER exceeds the BER threshold value, the Vdtc search algorithm fails to converge to a new Vdtc corresponding to Vdtc_opt and fails to update the transponder with a new optimum value of Vdtc corresponding to the new optical signal conditions. The transponder will therefore use a non-optimal value of Vref (i.e., a value determined for a previous set of optical data signal parameters) to recover data from the optical signal and BER will increase.
FIG. 11 further illustrates how a magnitude of change in an optical data signal in a communication system known in the art may be characterized by a corresponding effect on a calculated value of BER. For example, a relatively large magnitude of change in a value of OSNR, such as a change from a high value of OSNR to a low value or a change from a low value to a high value, is a change large enough to cause a value of BER to exceed a BER threshold value. A relatively large magnitude of change in a value of RXP for an optical data signal similarly causes a value of BER to exceed a BER threshold value.
Vdtc search algorithm failures may occur in a prior art communication system when the system is started up, although failures may also occur after start-up since large, rapid changes in OSNR and optical power are known to occur in optical data signals. What is needed is a system having means for dynamically mapping a value of Vref from values for Vdtc and optical power. What is further needed is a system that adjusts Vdtc to improve the efficiency of subsequent searches for optimal values of Vdtc after changes in optical signal power and OSNR.